The invention relates to apparatus for carrying out so-called haemofiltration or haemodiafiltration. In this process of medical treatment, which provides a substitute for the natural kidney function, the blood of the patient is passed along the surface of a filter membrane in an extracorporal blood circulation, and subsequently returned to the patient. A filtrate is produced by a negative pressure on the other side of the membrane, the filtrate inter alia including the urine-bearing substances which are normally to be excreted by way of the kidneys. At the same time, a substitute fluid is mixed with the blood, in accordance with the amount of filtrate produced, in order to maintain a balance in the respective volumes of fluid involved.
In order to produce a sufficient pressure differential between the two sides of the membrane, it is usual to operate with a reduced pressure on the filtrate side. For example, it is known from the literature (L. W. Henderson et al: Kinetics of Haemofiltration, Journal of Laboratory and Clinical Medicine 85 (1975), 372-391), to use an adjustable vacuum source which is not specified in greater detail and which sucks air out of a vessel into which the filtrate flows. In other known equipment for haemofiltration, tube pumps are used to produce the reduced pressure, the tube pumps sucking off the filtrate and conveying it away into a collecting vessel. The pump speed is automatically set by way of an electronic controller in such a way that either a given increase in volume per unit of time occurs in the collecting vessel, or a given preselected pressure is maintained at the input of the pump. In the latter process which is generally considered as being more advantageous, because it can be more easily used for different types of filters, without consideration of the transmissiveness thereof, it is possible to use either a continuous pressure control or a two-point pressure control. Both cases require measuring means for the pump input pressure, which have an electrical output either in the form of a switching means for switching the pump on and off (two-point control) or in the form of an analog signal generator for continuously controlling the pressure by way of an electronic controller which is connected upstream of the pump motor.
In the known apparatus, the total expenditure is considerable, comprising at least the pump, a pressure measuring means with an electrical output and the controller for the pump drive.